# Check your optics skills! Double refraction or trouble refraction

**Classical Mechanics**Level 3

A long cylindrical tube containing water is closed by an equiconvex lens of focal length \(20 \text{ cm}\) in air. A point source is placed along the axis of the tube outside it at a distance of \(50 cm\) from the lens. Find the distance from the centre of the lens where the image is formed.

Additional data :- \(n_\text{lens} = 1.5\) and \(n_\text{water} = \frac{4}{3} \approx 1.33\)

**Note**: In solving the question you may use the following approximation :- \(0.67 \approx \frac{2}{3}\).

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